1. Field of the Invention
The present invention relates to suspension systems for vehicles, and, more particularly, to suspension systems for off-road vehicles.
2. Description of the Related Art
Vehicles in the form of off-road machines can take many forms, such as agricultural tractors, harvesters and sprayers, construction backhoes, and forestry feller/bunchers. In the case of an agricultural tractor, a chassis typically supports front and rear laterally extending axles having wheels rotatably affixed at the ends of the axles. Rear axles are commonly rigidly connected to the chassis, and there is no suspension between the rear axle and the chassis. Tire deflection alone provides a cushioning between a rough surface and the chassis. In many tractors, the front wheels are driven in addition to the rear driven wheels, typically known as mechanical front wheel drive (MFD or MFWD). Economics and simplicity typically dictate that rigid front axle assemblies are used in lieu of an independently articulated front suspension. Front axles are typically pivotally attached to the front of the chassis to rotate, transversely with respect to the longitudinal axis of the chassis, around a pivot axis located proximate to the longitudinal middle of the axle. With this axle structure, when one front wheel is raised to overcome an obstacle the other front wheel goes down the same distance.
The productivity of an agricultural tractor can be increased by enabling faster travel speeds in the field and on the road. Limiting factors to the travel speed of an agricultural tractor when traveling over rough surfaces are operator comfort and wheel traction. Faster travel speeds highlight shortcomings in the conventional pivotally attached rigid front axle, especially when both wheels simultaneously encounter a similar obstacle, such as a ditch. When both wheels must move in the same direction to traverse an obstacle, the entire front end of the vehicle is forced to move in the same vertical direction.
Tractor front axle suspensions can solve these problems by enabling the entire axle to move in relation to the chassis. By damping such motion traction and operator comfort may be increased both of which may contribute to increased productivity. Additionally, front suspension systems for tractors provide better high-speed handling characteristics during high-speed operation on roads. The addition of more sophisticated actuators and control systems to these suspension systems further extend the capabilities of the tractor and provide additional improvements in vehicle ride and handling.
As a suspension system moves up and down, the degree to which the axle can roll about the pivot axis varies so that the wheels at the end of the axle do not hit the chassis, sheet metal or other parts of the vehicle. When the axle is further from the chassis the maximum roll angle can be greater, and when the axle is closer to the chassis the maximum roll angle must be less.
A front suspension system as describe above typically includes an axle carriage which is mounted to or part of the chassis, and the rigid front axle is mounted to the axle carriage. A suspension arm is pivotally mounted to the chassis at the rear of the arm, and pivotally coupled with the axle at the point of connection of the MFD drive shaft with the axle. As the axle moves up and down, the suspension arm pivots about the rear pivot attachment. It is known to provide two pairs of roll angle stops for limiting the roll angle of the axle when the axle is at a raised or lowered position relative to the axle carriage. When the axle is at a fully raised position (closest to the axle carriage), one pair of roll angle stops is between the axle and axle carriage to limit the roll angle of the axle. On the other hand, when the axle is at a predefined position in the suspension stroke such as the fully lowered position (furthest from the axle carriage), the other pair of roll angle stops is between the axle and suspension arm to limit the roll angle of the axle. High loads can be placed on the suspension arm when the roll angle stop of the axle hits the suspension arm, and thus the suspension arm is typically built very robust to withstand the loads. A heavily built suspension arm results in more weight and expense.
What is needed in the art is a front suspension system for an off-road vehicle which effectively limits the roll angle of the axle, depending on the height of the axle relative to the axle carriage.